During the operation of nuclear systems, particularly nuclear power plants, radioactive waste gases develop which contain, inter alia, krypton and xenon nuclides. These radioactive waste gases must not be discharged into the atmosphere at will since the regulating authorities require the observance of maximum permissible values of radioactivity. The discharge of these gases into the atmosphere must thus be delayed for a period of time sufficient for the radioactivity to drop to a value below the maximum permissible values.
It is known (for example "Kerntechnik" 13 (1971) page 205 and Ind. Eng. Chem. 51 (1959) page 1467) that these requirements can be met if the waste gas which is contaminated with radioactive noble gases is conducted over activated carbon delay paths where the noble gases remain substantially longer than the inactive carrier gas, for example, air, because of the adsorptive forces of the activated carbon. Such delay systems generally operate at temperatures around 20.degree.C and at pressures of about or slightly less than 1 atmosphere absolute.
It has been found, however, that the long-lived radio-nuclides, particularly Kr.sup.85 with a half-life of 10.7 years, are decomposed only insignificantly by such a delay. The danger thus exists that with an increase in the number of nuclear systems the concentration of Kr.sup.85 in the atmosphere will continuously increase.
For this reason processes have been developed in which the radioactive noble gases krypton and xenon are separated from the exhaust or waste gas mixture and are stored in a highly concentrated form in a radiation-proof manner. A number of such processes which will be called separating or enrichment processes hereinafter are known.
Thus, for example, it is known that the radioactive noble gases may be separated from the waste gases from nuclear power plants and fuel element processing plants by liquefaction and fractionated distillation of the waste gases. Furthermore, processes are known in which the noble gases are separated from the waste gases by absorption in solvents or by the use of permselective membranes. Finally, processes have been proposed in which the noble gases -- particularly the long-lived Kr.sup.85 -- are separated from the waste gases adsorptively, preferably by way of adsorption on activated carbon and are stored in a radiation-proof manner. Generally the adsorbers are operated at low temperatures (-40.degree. to -195.degree.C), which makes them expensive to maintain. For regeneration purposes, the charged activated carbon is either evacuated (BNL Report No. 689 (T-235), Brookmeade National Laboratory, September 1960), or an inert rinsing gas, preferably helium is conducted through the adsorption/desorption reactors (Industrial Chemist 39, (1963) page 358). In the former cases it is necessary, however, to increase the temperature to higher than +100.degree.C in order to achieve complete desorption of the noble gases. In the latter case the rinsing gas containing the desorbed fission gas components krypton and xenon can be conducted through a further, larger activated carbon column where krypton and xenon are separated from one another as in a gas chromatograph. This however produces an additional dilution by the carrier gas (helium), which must be removed from the desorption gas fraction containing the krypton and xenon in a further low temperature adsorber.
German Patent Application P 2210264 filed 3. 3. 1972, the contents of which are incorporated by reference, discloses a further process for separating and recovering mostly radioactive krypton and xenon nuclides from waste gases containing such nuclides, in which an adsorption of krypton and xenon to activated carbon is effected in a reactor until there is a krypton breakthrough at the output end of the reactor and thereafter the adsorption medium is regenerated in three stages. A great advantage of this process compared to the above-mentioned methods is that a special combination of pressure reduction and use of rinsing gas eliminates the need for low temperatures and temperature changes to produce sufficient enrichment factors. The rinsing gases may be, for example, air, nitrogen or helium. CO.sub.2 or other gases which can easily be condensed out are suitable as rinsing media at normal or not very low temperatures (to about -20.degree.C). A further significant advantage compared to the prior methods is finally that a separation of the krypton nuclides from the xenon nuclides can be effected during the regeneration of the adsorber, i.e. in the same operating step and without the use of an additional chromatographically operating activated carbon vessel. The method according to the above mentioned patent application will be explained in further detail in the description of the drawing figures since it can be used with advantage in connection with the present invention.
All of the separating or enrichment processes discussed above have not been able to gain major significance in practice and, in particular, the safe storage of concentrated radioactive gases raises great difficulties. Additionally, the regulating authorities have thus far not required that the long-lived Kr.sup.85 be quantitatively separated from the waste gases of the nuclear power plants. Thus activated carbon delay paths or columns are usually employed in practice.
The known activated carbon delay paths must be designed at the present time so that delay times of about two to three days for krypton and of about 30 to 50 days for xenon are achieved. The quantities of activated carbon required for this purpose usually amount to more than 100m.sup.3 for the usual gas quantities at normal operating pressures and temperatures.
Since the discharge conditions for radioactive substances into the environment will be more strict in the future, this would involve an even further increase in the capacities of the activated carbon systems which should be avoided for reasons of safety and costs, since the costs for enclosed space in nuclear power plants are very high due to the radiation protection measures involved. At present such costs amount to about 250 German marks per m.sup.3 of such enclosed space.
In principle, the possibilities for decreasing the delay systems are either to cool the delay path or to increase the operating pressure in the delay system. In the former case, the expenses involved for the cooling systems required for this purpose, are substantially greater than the savings due to reduction of structural volume. Regarding the possibility of increasing the operating pressure, this produces special problems regarding the tight sealing and safety of such systems which also requires considerable expense.